Carb Size Formula

[mach70]

Carb Size Formula


Carb CFM = ((Cubic Inches X Max Revs) / 3456) X VE %)

VE= Volumetric Efficency. Most Street Engines 75-85%
Race Emgines 90-95%

So for a 289 it looks like this

Carb CFM = ((289 X 6000) / 3456) X 85%)

Carb CFM = ((1734000) / 3456) X 85%)

Carb CFM = ((501.7361) X 85%)

Carb CFM = 426

[IGALOP]

For 2 barrel Motorcraft/Autolite carbys the usual venturi size for a c-code engine is 1.14 inches. This is marked on the side of the float bowl inside a circle.

I have a 1.21 inch venturi Autolite carby that I will fit and this will increase flow from 300 cfm to 351 cfm.
Some of these larger versions were fitted to early Fairlanes, Falcons, Broncos etc.
See below for 2 barrel venturi sizes in inches and relative flow in cfm.

0.98"=190 cfm
1.01=240
1.02=245
1.08=287
1.14=300
1.21=351
1.23=356
1.33=424